There is a deliberateness to this word. I think. The sense I have is of a person who made the decision to become austere as opposed to having poverty or austere circumstances thrust upon them. Or of someone who is stern in appearance.

All the references I have looked at give the first sense of the word as you see it. Austerity is more an aura than a condition. I think it is a poor word to describe someone who is poor. It is a good word for someone who is cheap and grasping.

Austere always implies to me the high end of the social scale."Let them eat cake" is the poster child for austerity. I agree it can be a decision, but one can be raised to an austere haughtiness, a sometime synonym, particularly in a family of perceived royalty or old money. I'm not sure why it also can be used of poverty, though in many contexts it seems so.

As it comes from the Greek for harsh, rough, and bitter, I can see how it could apply to poverty.

Here is a nice write-up from dictionary.com:

4. Austere, bleak, spartan, stark all suggest lack of ornament or adornment and of a feeling of comfort or warmth. Austere usually implies a purposeful avoidance of luxury or ease: simple, stripped-down, austere surroundings. Bleak adds a sense of forbidding coldness, hopelessness, depression: a bleak, dreary, windswept plain.Spartan somewhat more forceful than austere, implies stern discipline and rigorous, even harsh, avoidance of all that is not strictly functional: a life of Spartan simplicity.Stark shares with bleak a sense of grimness and desolation: the stark cliff face.

Life is like playing chess with chessmen who each have thoughts and feelings and motives of their own.